Tumors invoke immunosuppression in the host by inducing an altered profile of cytokine expression. The crucial regulatory cytokine IL-12 operates as a bridge between innate and adaptive immunities. It is produced by macrophages and dendritic cells, and acts upon NK and T cells to induce their production of IFN-gamma, driving T cells to develop Th1 responses. IL-12 has also been recognized to exhibit strong anti-tumor properties. Therefore, by blocking IL-12 expression, the tumor might guarantee its own survival. The possibility of a tumor-induced downregulation of IL-12 gene expression as a tumor survival strategy has been scarcely explored. The long-term goal of my research is to clarify the role of macrophages in mammary tumor development. We have shown that the mouse mammary tumor D1-DMBA-3 directly produces factors, and induces B cells and macrophages from the host to produce factors that inhibit the production of IL-12 by peritoneal elicited macrophages from tumor-bearing animals. I hypothesize that these tumor-associated factors, in particular the tumor-produced phospholipids phosphatidylserine (PS) and the tumor-induced pro-inflammatory cytokine IL-6, exert their immunosuppressive effects at least in part by downregulating IL-12 gene expression at the transcriptional level in macrophages, resulting in evasion of the host's immune defenses and enhanced tumor growth. The objective of the present application is to reveal the molecular mechanisms of the D1-DMBA-3 tumor-mediated inhibition of IL-12 gene expression in macrophages from tumor-bearing animals. I plan to test my hypothesis by pursuing the following specific aims: 1) To elucidate the mechanisms by which the presence of the D1-DMBA-3 tumor, and specifically the tumor-derived factor phosphatidylserine (PS) and the tumor-induced cytokine IL-6, downregulate the production of IL-12 in macrophages from tumor-bearing animals as well as in macrophages from normal mice pretreated with these tumor-associated factors, respectively;2) To investigate the consequences of manipulating the expression of the critical IL-12-inducing transcription factors NFkB and C/EBP on IL-12 expression in macrophages from normal and tumor-bearing animals, as well as in macrophages from normal mice pretreated with PS and IL-6;3) To determine the effects of modulating PS and IL-6 in vivo on IL-12 production and mammary tumor progression. These studies constitute a first step towards the goal of manipulating the levels of IL-12 in tumor hosts, leading to appropriate immune activation.